Fix bug in ST create, fix documentation for ST

[rebecca-makar]

Without Flag:

PS C:\Users\remakar\azure-powershell-pr\artifacts\Debug\Az.HPCCache> New-AzHpcCacheStorageTarget -ResourceGroupName test_rg -CacheName test_cmdlet_cache -Name test_cmdlet_st  -StorageContainerID "<ID>" -Junction @(@{"namespacePath"="/cache/storage/targets/test_cmdlet_st";"targetPath"="/"})

Confirm
Are you sure you want to create HPC cache storage target 'test_cmdlet_st'?
[Y] Yes  [N] No  [S] Suspend  [?] Help (default is "Y"): Y
New-AzHpcCacheStorageTarget : Need CLFS or NFS flag.
At line:1 char:1
+ New-AzHpcCacheStorageTarget -ResourceGroupName test_rg -CacheNam ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo          : CloseError: (:) [New-AzHpcCacheStorageTarget], Exception
+ FullyQualifiedErrorId : Microsoft.Azure.Commands.HPCCache.NewAzHpcStorageTarget

With Wrong flag (NFS FLAG ON CLFS ST):

PS C:\Users\remakar\azure-powershell-pr\artifacts\Debug\Az.HPCCache> New-AzHpcCacheStorageTarget -ResourceGroupName test_rg -NFS -CacheName test_cmdlet_cache -Name test_cmdlet_st  -StorageContainerID "<ID>" -Junction @(@{"namespacePath"="/cache/storage/targets/test_cmdlet_st";"targetPath"="/"})
New-AzHpcCacheStorageTarget : Parameter set cannot be resolved using the specified named parameters. One or more parameters issued cannot be used together or an insufficient number of parameters were provided.
At line:1 char:1
+ New-AzHpcCacheStorageTarget -ResourceGroupName test_rg -NFS -Cac ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo          : InvalidArgument: (:) [New-AzHpcCacheStorageTarget], ParameterBindingException
+ FullyQualifiedErrorId : AmbiguousParameterSet,Microsoft.Azure.Commands.HPCCache.NewAzHpcStorageTarget

With correct (-CLFS):

PS C:\Users\remakar\azure-powershell-pr\artifacts\Debug\Az.HPCCache> New-AzHpcCacheStorageTarget -ResourceGroupName test_rg -CacheName test_cmdlet_cache -Name test_cmdlet_st123 -CLFS -StorageContainerID "<ID>" -Junction @(@{"namespacePath"="/cache";"targetPath"="/"})

Confirm
Are you sure you want to create HPC cache storage target 'test_cmdlet_st123'?
[Y] Yes  [N] No  [S] Suspend  [?] Help (default is "Y"): Y

Name              : test_cmdlet_st123
Id                : <ID>
TargetType        : clfs
ProvisioningState : Creating
Junctions         : {Microsoft.Azure.PowerShell.Cmdlets.HPCCache.Models.PSNamespaceJunction}
Nfs3              :
Clfs              : Microsoft.Azure.PowerShell.Cmdlets.HPCCache.Models.PSHpcClfsTarget

In new-azhpccachestoragetarget it was throwing the wrong exception if the storage target already exists, it was throwing upgradeCache instead of storageTargetAlreadyExists.

Fix examples in help files. -Junction instead of -Junctions

Confirm
Are you sure you want to create HPC cache storage target 'test_cmdlet_st'?
[Y] Yes  [N] No  [S] Suspend  [?] Help (default is "Y"): Y
New-AzHpcCacheStorageTarget : The storage target 'test_cmdlet_st' already exists for HPC cache 'test_cmdlet_st'.
At line:1 char:1
+ New-AzHpcCacheStorageTarget -ResourceGroupName test_rg -CacheNam ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo          : CloseError: (:) [New-AzHpcCacheStorageTarget], Exception
+ FullyQualifiedErrorId : Microsoft.Azure.Commands.HPCCache.NewAzHpcStorageTarget